A torsion spring is a type of spring that stores mechanical energy when a twisting force (torsion) is applied. These include torsion bars where the torsion is resisted by shear stresses, and spiral torsion springs wherein the torsion is resisted by bending stresses about the axis of their curvature. A spiral torsion spring has a coil portion with one or more coils usually forming a generally circular annulus with a coil axis and a transverse diameter generally perpendicular to the coil axis. When a torque is applied to a spiral torsion spring, an angular displacement between the first and second loading points is created, the coil deflects, and the material from which it is made is placed under stress. Spiral torsion springs may form a helix; a spiral; be flat, conical, spherical or volute in shape; or be less than a full coil; and are differentiated from compression and extensions springs by their application in resisting torsion.
A torsion spring can be linear or non-linear. In a linear torsion spring, the applied torque is directly proportional to the angular displacement via an unchanging variable called the spring rate. That is, the ratio between applied torque and angular displacement is constant. In a non-linear torsion spring, the applied torque and angular displacement are not proportional. There are two types of non-linear torsion springs, hardening and softening. In a hardening torsion spring, the ratio between applied torque and angular displacement grows such that with the application of an additional degree increase in angular displacement, more additional torque will be required than would be required for a linear spring. For example, in the simplest hardening spring, the ratio between applied torque and angular displacement equals k1+k2·Θ where k1 and k2 are constants and Θ is the angular displacement. In a softening torsion spring, the amount of applied torque grows less than linearly.
Some systems provide a torsion spring having a series of holes along the spring axis that can be fixed to an end plate used to apply torque to the spring. This scheme allows a one-time adjustment of the length of the spring and thus its effective spring rate. While useful for matching the torque range to a particular application, the spring's applied torque is still directly proportional to the angular displacement because the spring's length does not change with applied torque.
Torsion springs are useful in a wide variety of applications including supplying power to mechanical devices such as clocks and toys, absorbing shock during motor startup and the like. While most torsion spring applications employ linear torsion springs, non-linear torsion springs are more suitable for some applications. For example, compound hunting bows, truck suspensions, and positioning applications in conjunction with servo controls where avoiding mechanical overshoot is desired, benefit through the non-linear properties while linear torsion springs generally do not help address the mechanical deficiencies.
Torsion springs provide for a relationship between torque and angular displacement. They can be used to simulate the relationship between torque and angular displacement of an existing mechanical system. Linear torsion springs can only replicate systems whose relationships are linear, while systems whose relationships are non-linear can only be replicated by non-linear torsion springs. As an example, a truck suspension employing a non-linear torsion spring is replicating an alternative system of compression springs, dampers, and linkages. Another system that has a non-linear relationship between torque and angular displacement is the human knee during ambulation with the torque provided by the quadriceps. In persons with deficient quadriceps, ambulation becomes difficult or impossible due to an inability to provide the necessary torque at sharper knee angles (larger angular displacements). Therefore, there exists a need for a system that can provide the non-linear relationship between torque and angular displacement of the human knee during ambulation. Further, there exists a need for a means of providing the non-linear relationship between torque and angle of the human knee during ambulation to those whose quadriceps are unable to provide that relationship.